This invention relates generally to a method and apparatus for determining both the pore-size characteristics and integrity of porous structures, particularly membrane filters and fabricated devices incorporating such filters. Specifically, this invention relates to a method and apparatus for determining the pore-size characteristics and/or integrity of a membrane filter based upon diffusion testing involving the use of test measurement gases and photo-acoustic generation/detection techniques.
Presently, the pore-size characterization and determination of integrity for membranes and filters, in general, are performed using procedures which are referred to as, among other things, "air-flow porosimetry", the "bubble-point test" or "bubble-point determination", and the "diffusion test". In addition, hydrophobic membranes, specifically, can also be characterized and tested by procedures referred to as, among other things, the "water intrusion-pressure determination" and the "water flow test" or "water intrusion test". U.S. patent application Ser. No. 08/105,525, now U.S. Pat. No. 5,477,155, assigned to the same assignee as the present application, describes a variation of the water intrusion method in which electrically conductive liquids are used to identify the pressure at which the liquid intrudes into and through the pores of the membrane,
The bubble-point test and air-flow porosimetry utilize a liquid which spontaneously wets the membrane in question to create a barrier to gas flow. Subsequent attempts to displace the wetting liquid with a gas require that the gas pressure be elevated to some critical level dependent on the size of the pores, or the size of defects, if present, in order to overcome the surface-tension forces holding the liquid in the pores. The equation for this critical pressure, defined as the bubble-point pressure, is a variation of the Young-Laplace equation for capillary pressure drop, in this application often called the Washburn equation: EQU PBUBBLE POINT=4 K .sigma. cos (.theta.)/d (Equation 1)
where;
PBUBBLE POINT=bubble-point pressure PA1 K=the pore perimeter (shape) correction factor PA1 .sigma.=surface tension of the liquid PA1 .theta.=contact angle of the liquid against the solid PA1 d=the diameter of the pore
Equation 1 is rarely actually used to quantitatively calculate a pore size from empirical bubble-point data, since the pore perimeter correction factor, K, is rarely known independently. Instead, since this equation indicates that the bubble point is inversely related to the pore diameter, it is used to qualitatively rank the relative pore size of membranes according to their bubble-point pressures. Further, since particle retention efficiency is related to the pore size, Equation 1 is also used to justify an empirical correlation between the retention efficiency of membranes of various pore sizes to their bubble points. Membrane manufacturers and users have taken advantage of this retention vs. bubble point relationship to identify the critical bubble point required for a desired level of retention, and membrane filter users conduct bubble point determinations to confirm that the filter in question is integral and of the appropriate pore size. Integral refers to the fact that the filter element will have the desired level of retention and contains no defects or large pores that diminish this desired level of retention.
Air-flow porosimetry and a visual version of the bubble-point test for membrane samples are described by ASTM Method F316-86. In general, the bubble-point test is performed by pre-wetting the membrane with an appropriate liquid and mounting the membrane in a specially designed holder which allows a visually observable layer of liquid to be placed on the downstream, i.e., in this configuration, upper side of the membrane. In the case of a bubble-point test of an enclosed filter, the filter is flushed with the liquid to wet the membrane. The pressure of air or other gas on the upstream side of the membrane is then increased, and the downstream liquid layer or the outlet from the enclosed filter is observed for the formation of continuous streams of bubbles. The pressure at which these bubbles first appear is called the bubble-point pressure of the membrane.
For relatively large membrane filters, which as discussed below experience significant rates of gas diffusion at pressures below the bubble point, a more analytical method is used to determine the bubble-point pressure. In this case, the rate of flow of gas through the filter is measured as a function of the imposed gas pressure, and the pressure at which the flow makes a transition from relatively low flow rates, which are indicative of diffusion only, to significantly higher flow rates, which are indicative of bulk gas flow through pores or defects, is referred to as the bubble-point pressure of the filter. This method has been described in a presentation by Knight and Badenhop at the 8th Annual Membrane Planning Conference, held Oct. 15-17, 1990.
Porosimetry is used to determine the relative pore-size distribution of a membrane or membrane filter. In this procedure, the flow rate of gas through a pre-wetted membrane at a particular gas pressure is divided by the flow rate of gas through an initially dry identical membrane at the same pressure. The resulting mathematical ratio, R, is plotted as a function of imposed pressure, and the first derivative of this function, dR/dP, yields a bubble-point pressure distribution, which, via the bubble-point equation (Equation 1), also indicates the relative distribution of pore sizes.
The diffusion test is used primarily for relatively large filter areas and indicates whether or not the filter is integral by measuring the gas flow rate of a test gas through the filter when exposed to a constant upstream gas pressure equal to, or slightly below, the minimum bubble-point pressure required for the filter. Similar to a bubble-point test, the filter is pre-wet with an appropriate liquid. At a properly selected test pressure, the measured flow rate of the test gas will be relatively low (indicative of diffusive as opposed to convective flow) when the filter is integral and of the pore size appropriate for the selected test pressure. The source of gas flow through an integral filter at pressures below the actual bubble point of the filter can be attributed to dissolution of gas into, diffusion through, and re-evaporation from the liquid filling the pores, without forcing the liquid out of the pores. In such a test, a filter with an undesirable large pore size or with a defect will exhibit relatively large gas flow rates of the test gas as a result of the test pressure being in excess of the bubble-point pressure attributed to the defect.
In practice, both diffusion and bubble-point test measurements are conducted in two ways, namely a direct measurement of mass flow of the gas or an indirect pressure decay measurement. In the mass flow measurement, the flow rate of the transport gas is measured directly after steady state is achieved. A pressure decay measurement is performed by isolating the volume upstream of the membrane after achieving the desired test pressure and monitoring decay of pressure as the gas occupying this volume is depleted by either diffusional or convective transport through the wetted membrane. A single measurement is generally made after 2 to 5 minutes. The sensitivity of both techniques is limited by the solubility of the gas in the wetting medium since this solubility controls the background diffusion or noise level. In a defective element, the increased gas flow associated with defects must be quantifiably above this background diffusion level.
A more sensitive test can be designed based upon the principles of dynamic residence time distribution (RTD) measurements. This is a standard technique used to determine the hydrodynamic and fluid mixing properties of vessels. As applied to membrane filter integrity testing, a diffusion test would be performed in the conventional manner. However, a second detector gas would be used to perform the measurement. In its simplest form, the upstream surface of the liquid-filled membrane is contacted with a gas, such as air, which is used in this case as a carrier gas. A second tracer gas, such as sulfur hexafluoride, either pre-mixed with the carrier gas or alone, is injected into the air-carrier gas upstream of the membrane surface and elevated to the desired trans-membrane test pressure. The sulfur hexafluoride is added permanently to the carrier gas such that the total trans-membrane pressure remains constant. Alternatively, the tracer gas can be injected as a pulse into the carrier gas which is elevated to the test trans-membrane pressure. The concentration of the tracer gas is then monitored as a function of time on the downstream side of the membrane. The transport of the tracer gas across an integral membrane unit is controlled by diffusion as described above. However, in a non-integral membrane unit, the test pressure will exceed the bubble point of large defects, the pores are evacuated of liquid and the transport of the tracer gas is governed by convective or bulk flow. Since bulk flow will transport gas much faster than diffusion, use of the RTD measurement technique results in a significant sensitivity benefit over the quasi-steady state diffusional measurement conventionally practiced. It should be noted that this process can be conducted at non-constant trans-membrane pressure.
The RTD technique was first described as a method for integrity testing a cartridge membrane filter in the aforementioned presentation by Knight and Badenhop. In this presentation, a method is described in which a water-wetted membrane unit is challenged at a trans-membrane pressure near the characteristic bubble point of the membrane with air, the carrier gas. After steady state is achieved, the detector gas, sulfur hexafluoride, is introduced into the incoming air stream while maintaining a constant applied pressure. The concentration of the sulfur hexafluoride is monitored in the volume downstream of the membrane cartridge. This method demonstrated sensitivity that greatly exceeded conventional diffusion techniques. However, the method described by Knight and Badenhop is severely limited by the analytical procedures employed. Grab-samples of the downstream gas are collected using an elaborate sample collection assembly intended to avoid sample contamination. The sulfur hexafluoride concentration is then measured off-line using an electron capture device that includes a gas chromatograph pre-treatment to remove moisture from the sample. The relatively long analysis time and re-equilibration time between individual batch samples conducted off-line inhibit this approach from having practical value as a routine test for determining integrity or pore size distribution.
Photo-acoustic spectroscopy is a well established technique for detecting trace quantities of gas (see Kreuzer 1971, J. Appl. Phys. 42 p2934-2943; Kreuzer and Patel 1971, Science 173, p45-47). In this conventional technique, a gas sample is irradiated by a chopped laser beam. When the laser wavelength coincides with an absorption line of the gas to be detected, the absorption of the radiation produces temperature and pressure increases in the gas. The subsequent re-emission results in pressure oscillations that are detected with a microphone. In the conventional technique both the sample gas and the microphone detector are co-located within a closed cell. This photo-acoustic technique has been extended by remoting the microphone detector away from the sample chamber as described by Brassington 1982, J. Phys. D. Appl. Phys. 15, p219-228, thus enabling remote leak detection.
Several device configurations are described in the prior art for conducting photo acoustic detection. For example, U.S. Pat. No. 4,557,603 to Oehler et al. discloses the use of a monochromator to vary the wavelength of the light, and U.S. Pat. No. 4,622,845 to Ryan et al. discloses the use of a pulsed infrared light source and an acousto-optical tunable filter to provide the desired wavelength of light.
U.S. Pat. No. 5,161,408 to McRae et al. discloses an apparatus that uses a monochromatic laser of known fixed wavelength which is strongly absorbed only by sulfur hexafluoride (SF.sub.6) gas. The laser beam employed by McRae et al. scans a two dimensional field in the test area of a container or other gas or liquid-tight component. Any trace SF.sub.6 gas excited by the scanned laser beam will produce an audible sound that is detected by a microphone. A discriminating electronic circuit is disclosed to process the electronic signal from the microphone. The prior art with respect to photo-acoustic detection has typically addressed various devices for applying external photo-acoustic techniques to the determination of leak detection from systems intended to be gas-tight or liquid-tight. In these systems the measurement conditions are binary in nature, that is the system is only concerned with distinguishing between a normal condition where there is zero flow of tracer gas and an abnormal condition where some discernible flow is detected. The concentration level of the gases being detected is of minimal importance in such systems. Oehler et al. in U.S. Pat. No. 4,740,086 makes reference to measuring the permeability of gas-permeable elements using photo-acoustic detection with their apparatus; however, bulk gas permeability measurement is altogether different from measuring the pore size or determining the integrity of a porous membrane element which requires discriminating between holes (pores) of different sizes. Moreover, no mention is made in the prior art of making photo-acoustic detection measurements in a moisturized gas. Thus in these systems the background signal, assuming that the laser is specifically tuned only to the tracer gas, is negligible. Consequently, in these systems the interpretation of the signal produced by the photo-acoustic effect is greatly simplified.